Why do I get suck back at a stable temperature

[wsmith1625]

I'm just curious if anyone out there can explain to me why I get suck back at a stable temperature when I cold crash. Let me explain. I ferment and cold crash in a converted wine fridge that is controlled by an Inkbird temperature controller. My fermenter is a converted Fermonster with ball lock posts connected to the lid.

When I initially start cold crashing my beer, I like to drop it down in 5 degree increments. Each time I check the blowoff tube to see if I'm getting suck back. When I see Starsan in the tube, I disconnect the gas post and apply a little co2 to pressurize the fermenter. When I reconnect the blowoff tube, the pressure clears the tube and then equalizes. I continue this process until I get to my target temperature.

The suck back when dropping the temperature is expected, but why do I continue to get suck back after the target temperature is reached. My understanding is that as the beer cools, it contracts creating negative pressure in the fermenter. If I'm at my target temp, the beer should already be contracted, so I wouldn't expect additional suck back. The temperature controller has a 1 degree differential so it's not warming up very much before calling for another cooling cycle.

I'm sure it's all normal and expected behavior, but I'm hoping someone with a little more understanding could explain this to me.

NOTE: I'm not looking to change my cold crash method, it works fine for me. Just want to get educated on what's happening. Thanks!

 

[doug293cz]

As beer (or just water) gets colder, it will absorb more CO2 at the same CO2 partial pressure (14.7 psi absolute in your case, but since a gauge reads the difference between atmospheric pressure and vessel pressure, your gauge reads 0 psi.) So, when you cold crash, first the pressure drops just due to the temp change of gas in the headspace, but more CO2 starts absorbing into the beer due to the colder temperature. This will continue until the CO2 partial pressure is in equilibrium with the level of CO2 dissolved in the beer. It is possible to calculate just how much CO2 will be absorbed due to any temperature change, and it turns out that for a typical cold crash temp, the amount of CO2 that can be absorbed by the colder beer is way more than the volume change just due to the temp change.

Brew on

 

[wsmith1625]

That makes perfect sense! CO2 absorption never occurred to me. Thanks for that explanation.

 

[Bobby_M]

Yeah, it's just so counterintuitive when you see it happen. When I was experimenting with the cold crash guardian (bladder bag) full of CO2, I got to visibly watch it happen. The initial crash would deflate the bladder about half way and then over the next 36 hours, the rest of it would deflate. It lead me to increase the bladder size from 1 gallon up to 2.5 gallons to give more time at the cold temp.

 

[Deadalus]

I played this game recently too lol. I cold crashed and was able to keep adding a little CO2 to push out the starsan creeping up the blowoffs until the temp stabilized. I was fiddling on and off with the blowoff valve as well. Then I left the blowoff tubes' valves open. I'd come back in the morning or evening and starsan kept creeping back up the tubes. It almost made it to the top once!

 

[k-daddy]

I get suck-back with 5 lbs. co2 pressure in the fermenter (even after temperature stabilization). Never been able to figure out why.

 

[doug293cz]

I get suck-back with 5 lbs. co2 pressure in the fermenter (even after temperature stabilization). Never been able to figure out why.

This seems highly implausible. There must be something unusual about your configuration. Can you provide a detailed description and photos of the fermenter and CO2 venting path that this occurs with?

Brew on

 

[MehNahMehNah]

Could it be related to barometric pressure rising/falling? Just a thought.

 

[day_trippr]

Not a chance. I attach a constant 0.4psi CO2 supply to my fermentors before initiating crashes and there is never a hint of suck back...

Cheers!

[edit] That was a rather nonsensical statement considering I never have a liquid blow-off connected to begin with All the CO2 from fermentations is used to purge kegs, and when that's done I remove the purge line so the fermentors have just the CO2 supply connected.

 

[k-daddy]

Could it be related to barometric pressure rising/falling? Just a thought.

Yes, it seems impossible but it happens. No pictures but this is my setup... Spike CF-5 fermentor with Blichmann Spunding valve https://www.blichmannengineering.com/product/tri-clamp-spunding-valve/?sync-done that sits on top of a hop bong and a butterfly valve under that. The blow-off tube goes into the Starsan. Post fermentation (and stable temp), the Starsan creeps up the tubing, even with pressure on the spunding valve. The co2 tank is set to 8 lbs. and the spunding valve set to 5 lbs. When I slowly open up the spunding valve, the Starsan retreats back to the well so I know there is pressure. A real head-scratcher.

 

[Deadalus]

Yes, it seems impossible but it happens. No pictures but this is my setup... Spike CF-5 fermentor with Blichmann Spunding valve https://www.blichmannengineering.com/product/tri-clamp-spunding-valve/?sync-done that sits on top of a hop bong and a butterfly valve under that. The blow-off tube goes into the Starsan. Post fermentation (and stable temp), the Starsan creeps up the tubing, even with pressure on the spunding valve. The co2 tank is set to 8 lbs. and the spunding valve set to 5 lbs. When I slowly open up the spunding valve, the Starsan retreats back to the well so I know there is pressure. A real head-scratcher.

If your CO2 is set to 8 psi and the spunding valve to 5 psi, all your CO2 would bleed out of the tank through the spunding valve.. I think you may have written that backwards. Even then, if your blowoff tube has a valve and the valve is open, you'd be bubblng out the blowoff tube. I may be misunderstanding...

Now if your CO2 is attached to a carb stone, then you would have to account for wetting pressure and then the numbers are possible the way they were. I was familiar with wetting pressure myself but I though it was small. I did not calculate it on my carb stones, but it's around 5 psi it appears and I will need to directly get the values next time.

 

[doug293cz]

I suspect that what you are seeing is the effect of CO2 in the blow-off tube being absorbed into the liquid in the blow-off catch vessel. This will reduce the pressure in the blow-off tube, and liquid will climb up the tube.

Brew on

 

[k-daddy]

This seems highly implausible. There must be something unusual about your configuration. Can you provide a detailed description and photos of the fermenter and CO2 venting path that this occurs with?

Brew on

I spund @ 8lbs and capture fermentation co2 in the keg > Starsan bath.
Post fermentation, I remove keg, re-route blow-off tubing straight to Starsan well, CLOSE completely the Blichmann spunding valve and connect co2 tank to gas fitting (set to 5psi) for the DH/rest.
During that time, Starsan begins to creep UP the tubing towards the CLOSED spundiing valve.
Environment & fermenter temperature stabilized or very close.
Can't figure out what is causing this to happen so I keep an eye on it and when necessary, I open the spunding valve until the gas flows and pushes the Starsan back into the well, then close valve, repeat. I'd rather do that than disconnect the blow-off tubing until I can figure it out. I know, it sounds crazy to me as well.

FYI - blow-off tubing is 1/2" ID PVC (not silicone), about 3' in length.

Here's a link to the Spunding valve to put a picture to it (gas imput, spunding valve and pressure gauge all incorporated). Spunding valve is a spring piston with an o-ring seal and pressure adjustment screw. https://www.blichmannengineering.com/product/tri-clamp-spunding-valve/?sync-done

 

[k-daddy]

I suspect that what you are seeing is the effect of CO2 in the blow-off tube being absorbed into the liquid in the blow-off catch vessel. This will reduce the pressure in the blow-off tube, and liquid will climb up the tube.

Brew on

It never made sense till your explanation. I never considered co2 absorption and pressure effects. So now, I can just disconnect the closed spunding/blow-off tubing and rest assured that o2 is not somehow creeping back into the pressurized fermentor. Learn something everyday! Thanks!